Acrylics and polyesters are common types of resins used in the manufacture of solvent borne thermosetting coatings. Acrylics are known for their excellent hardness, fast dry times, scratch, stain, chemical, and humidity resistance, and outdoor durability. They can be formulated to a very high glass transition temperature (Tg) and exhibit excellent light stability and hydrolysis resistance. Acrylic coatings, however, often lack flexibility and require high levels of solvent in the coating formulation to achieve a practical application viscosity. The high solvent requirement for acrylics makes it difficult to satisfy VOC (volatile organic compound) content regulations as mandated by various federal and state air quality organizations.
By contrast, polyesters are ideally suited to formulate low VOC content or “high solids” coatings and provide a good balance of performance properties. Polyesters, however, typically do not weather as well as acrylics. Blends of acrylic and polyester resins can be used to achieve high solids, solvent borne thermoset coatings with desirable properties. Coating compositions comprising blends of acrylic and polyester resins are known and are the subject of, for example, U.S. Pat. Nos. 4,076,766; 4,322,508; 4,338,379; 4,397,989; 4,751,267; and 4,716,200.
Aliphatic polyesters are frequently used to reduce viscosity and increase solids of thermosetting acrylic (abbreviated herein as “TSA”) coatings and are desirable as a primary film former in high solids coatings due to their ability to reduce VOC emissions. Coatings made from aliphatic polyesters are generally flexible but tend to be soft, resulting in poor resistance to solvents and chemicals, poor humidity resistance and poor outdoor durability.
To increase resin and coating solids, reduce viscosity, and lower VOC emission, aliphatic polyester resins can be blended with TSA resins. Unfortunately, the glass transition temperature (“Tg”) of the blend frequently decreases significantly as the polyester content increases. The lower Tg of the blend has an adverse impact on the desirable characteristics that the TSA resin imparts to the coating.
There is a need in the coatings industry for aliphatic polyester resins that, when blended with TSA resins, lower the viscosity while maintaining the Tg of the TSA resin in high solids thermosetting coating compositions.